The present invention relates to analog-to-digital converters of the type which alternately charge and discharge an energy storing means for converting an analog input signal into a digital signal. The charging and discharging of the energy storing means is done at different rates, at least one of such rates being dependent upon the amplitude of the analog input signal. By comparing the respective times of the charge and discharge cycles, a signal proportional to the amplitude of the input signal can be derived. This signal is converted into a digital signal by counting clock pulses during the charging and/or discharging cycles.
It is important to calibrate such A/D converters to preclude any offset error between the value of the analog input signal and the value represented by the digital output signal; i.e., to ensure that if the value of the input signal equals zero, then the value represented by the digital output signal also equals zero. In all prior art converters known to this inverter, this zero calibration requires precisely matched or adjusted component values. However, these component values are inherently subject to drift, thereby rendering unstable the zero calibration.
Some prior art converters include additional circuitry to automatically stabilize the zero calibration, generally by measuring the amount of drift in the critical component values and compensating therefor. The operation of these converters generally requires a periodic auto-zeroing cycle during which the drift is measured and compensated for.